Because of the low efficiency conversion, both ways. And storage issues. Huge energy loss when converting water, and lots of waste heat when hydro-generating. We haven't invented a machine yet that can convert electricity into enough hydrogen so that you are able to get almost the same amount of energy back from burning said hydrogen. Simply sticking an electrode into water generates 20% hydrogen and 80% waste byproducts (heat, smoke, galvanization, hydroplating, leaks in the tubing system, heat from all the resistance in the wires/water, slow disintegrating of the electrode, etc) And even if we did have the technology for making the above 99% efficient, turning the created hydrogen back into electricity is very inefficient still. And storing it, oh my. Putting it under pressure is expensive. Compressors that size eat terawatts of power. Or if you choose not to pressurize, then procuring enough giant containers will be the problem. Plus pumping it in/out all the time.﻿